1. Field of the Invention
The present invention relates to improvements in a planetary gear transmission disposed between an engine or motor and a drive wheel or wheels of a motor vehicle such as an automobile and a railway carriage.
2. Discussion of the Prior Art
Planetary gear type transmissions which have a plurality of automatically selected gear or speed positions are widely used for motor vehicles. An example of such a planetary gear transmission is disclosed in Publication Nos. 50-32913 and 51-3012 of examined Japanese patent applications, wherein a first, a second and a third planetary gear unit are disposed in series, so as to provide five forward-drive positions. This transmission has a comparatively simple construction, and provides a relatively wide range of speed reduction ratio. Further, the radial dimension of the transmission can be made comparatively small, while the gear ratios .rho. of the planetary gear units (.rho.=number of teeth of the sun gear divided by number of teeth of the ring gear) are held within a suitable range.
In the conventional planetary gear transmission as indicated above, however, a shifting from the 2nd-speed position to the 3rd-speed position or from the 1st-speed position to the 2nd-speed position requires disengagement of one clutch which has been placed in its engaged position, and engagement of another clutch which has been placed in its disengaged position. Since the disengagement and engagement of the two clutches take place substantially concurrently or within a very short time, it is difficult to adequately control the shifting operation. More specifically, the vehicle engine tends to race if there exists an excessively large time lapse between the moment at which one clutch has been disengaged and the moment at which another clutch has been engaged. On the other hand, simultaneous partial engagement cf the two clutches may cause the transmission to be more or less locked, generating an undesirable shifting shock. Therefore, the frictional coupling devices such as the clutches should be adequately controlled to assure a smooth shifting operation of the transmission without such racing and locking phenomena of the engine and transmission. However, adequately controlling the disengagement and engagement of two clutches is difficult to attain.
The above problem is serious or conspicuous particularly when a shifting of the transmission is effected with the engine running at a relatively high speed, that is, when the shifting is associated with one of the 1st-speed, 2nd-speed and 3rd-speed positions which have relatively high speed reduction ratios (speed of the input member of the transmission divided by speed of the output member).